The invention relates to a device for a voltage supply and, in particular, to a circuit arrangement for a temporary voltage supply of electrical systems.
In electrical systems, there are frequently safety-related devices which have to have sufficient energy available not only when normal switching off processes occur but also, for example, when the normal operating current supply or operating voltage supply fails, in order to be able to still carry out a final operating sequence and, therefore, be able to arrive at a defined, safe state.
An example of this is an electronically controlled parking brake system for commercial vehicles. If the parking brake is not activated in such a system when there is a failure of the electricity supply, for example as a result of a short circuit in the vehicle battery, there is, on the one hand, the risk that it will no longer be possible to park the vehicle safely. However, on the other hand, when there is a failure of the electricity supply, the parking brake cannot be activated simultaneously and automatically in all cases because if a vehicle has not yet arrived at a stationary state brake actuation could lead to an unstable driving behavior and to a strong deceleration without the brake lights lighting up, as a result of which the risk of an accident is significantly increased. Therefore, after the vehicle has arrived at a stationary state when the electricity supply has failed, such a parking brake must be capable of being activated once more by the driver so that the vehicle can remain secured against rolling away.
Until now, such problems have been avoided by use of redundant voltage supplies from at least two large-capacity energy stores, which are alternately safeguarded with respect to one another or by use of an independent emergency power supply, i.e. one which is arranged separately. However, the known solutions are disadvantageous in that a plurality of energy stores are expensive and complicated to implement and, in addition, are difficult to monitor, and a separate emergency power supply has to be heavily overdimensioned owing to permanent self-discharging, which also results in high costs and a high degree of structural complexity.
In addition, the required energy for carrying out a final operating sequence (a “final action”) of the parking brake is frequently small, with the result that the known solutions do not correspond to the actual requirements in any way.
The invention is therefore based on providing a temporary voltage supply, which makes available the energy required for a final operating sequence of an electric system in the event of a fault in a cost effective and simple way.
The invention is based on the idea that for an electrical system, a small amount of energy is sufficient to be able to carry out a final process which gives rise to a safe and defined end state. Such an amount of energy is made available from an auxiliary power supply which, by virtue of being arranged directly in a switching device with short line paths and dividing elements, is protected against self-discharging such that overdimensioning of the auxiliary power supply or a separate emergency power supply can be dispensed with.
In particular, such a temporary voltage supply for a load comprises a switching device with an auxiliary power supply, a control device which is supplied by the auxiliary power supply if an operating power supply fails, and a switching device which is actuated by the control device and which activates the load by way of the auxiliary power from the auxiliary power supply. The auxiliary power supply is configured in such a way that sufficient energy for an operating sequence, which is the last sequence after the failure of the operating power supply, is made available to the control device on request by actuating a switch.
The switching device preferably has a diode arrangement with a low cut-off current, which counteracts self-discharging of the auxiliary power supply. As a result, the auxiliary power supply can be dimensioned in accordance with the requirements of the control device to be supplied.
Furthermore, the diode arrangement is activated automatically if the operating power supply fails, with the result that self-discharging losses are minimized and the time during which the control device can be operated by way of the energy from the auxiliary power supply is prolonged.
In addition, the auxiliary power supply and the diode arrangement are advantageously arranged in the immediate vicinity of the switch, as a result of which an undesired voltage drop on relatively long line sections is avoided.
The auxiliary power supply is advantageously a capacitor, particularly preferably an electrolyte capacitor with a voltage of the order of magnitude of 40 V and with a capacitance of the order of magnitude of 3000 μF.
The self-discharging of the auxiliary power supply can be monitored by the control device, preferably by temporarily disconnecting the charge circuit of the auxiliary power supply and measuring the voltage which occurs at the auxiliary power supply after a predetermined time, or by sensing the time up to the time at which the voltage of the auxiliary power supply has dropped to a predetermined value.
The control device also preferably determines the available power of the auxiliary power supply by monitoring the voltage drop between the operating power supply and a voltage at the switching device, and calculating the charge current from the voltage drop between the operating power supply and the voltage at the switching device.
The auxiliary power supply is advantageously continuously connected to the operating power supply, but a defined charge state of the auxiliary power supply can be monitored by virtue of the fact that the auxiliary power supply is regularly disconnected from the operating power supply for a predetermined time by way of a switch in the control device.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description when considered in conjunction with the accompanying drawing.